Micromechanical component and method

ABSTRACT

A micromechanical component and a method for producing a micromechanical component are proposed, a hollow space and a region of porous silicon being provided, the region of porous silicon being provided for lowering the pressure prevailing in the hollow space.

FIELD OF THE INVENTION

The present invention is directed to a micromechanical component and toa method.

BACKGROUND INFORMATION

Getter materials of porous metals are generally known which are used forbinding gases in closed systems. The task at hand is to enclose a lowpressure in such a closed system, such as a hollow space, for example,this pressure approaching vacuum pressure. Getter materials can be usedto bind the entrapped gases and, in this way, greatly reduce thepressure. The disadvantage associated with the known getter materials isthat the process for producing such materials is not suited for asemiconductor manufacturing process. Another disadvantage associatedwith the known getter materials is that they are not suited forsemiconductors.

SUMMARY OF THE INVENTION

In contrast, the advantage of the micromechanical component and of themethod of the alternative independent claims is that the getter materialprovided is made of porous silicon. This porous silicon may easily beintegrated into an existing semiconductor process. Moreover, the poroussilicon is also very suited for semiconductors. It is also beneficialthat the porous silicon is provided as an inexpensive getter material.Porous silicon binds oxygen (O2), for example, by forming silicondioxide already in response to low temperatures. This eliminates theneed for heating the component to high temperatures, as, for example, ina high-temperature annealing process, in order to activate the gettermaterial in the form of porous silicon. In addition, it is advantageousthat the porous silicon may be easily integrated into semiconductorprocesses, such as CMOS, BCD and the like, for example, and that theporous silicon may be used as getter material, for example, whenencapsulating sensors, in order to minimize the enclosed pressure. It isalso advantageously provided in accordance with the present invention touse porous silicon to produce a large surface area, in particular of upto over 1000 m² per cm³, to achieve highly effective gettercharacteristics.

It is especially beneficial that a first substrate and a secondsubstrate are provided, an intermediate layer being provided between thefirst and the second substrate. In this way, using especially simplemeans, such as wafer bonding, it is possible to imperviously jointogether the first substrate and the second substrate, thereby producingthe hollow space. It is also advantageous that the first and the secondsubstrate are joined to one another in such a way that they arehermetically sealed at the intermediate layer. To this end, it isadvantageously possible in accordance with the present invention toretain the vacuum provided by the getter material in the form of poroussilicon, in the hollow space. It is also beneficial that a firstsubstrate and a membrane are provided, the hollow space being providedbetween the membrane and the first substrate, and the region of theporous silicon being provided in the first substrate. This makes itpossible to manufacture an absolute-pressure sensor, for example, in asimple manner, which delivers a precise measuring result already inresponse to low pressures to be measured, because the pressure to becompared is very low in the interior of the hollow space.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagrammatic sketch of the micromechanical componentaccording to the present invention.

FIG. 2 is a first specific embodiment of the micromechanical component.

FIG. 3 is a second specific embodiment of the micromechanical componentaccording to the present invention.

FIG. 4 is a third specific embodiment of the micromechanical componentaccording to the present invention.

FIG. 5 is a fourth specific embodiment of the micromechanical componentaccording to the present invention.

DETAILED DESCRIPTION

In the manufacturing of micromechanical components or micromechanicalelements, such as acceleration, rotation-rate, or pressure sensors, ahollow space having an enclosed vacuum is often required. In all of thefigures, reference numeral 10 denotes the hollow space. Such hollowspaces or volumes 10 may be produced in different ways. For example, arecess may be produced by etching. In a subsequent step, a secondsubstrate 30 is bonded to a first substrate that has been pretreated insuch a manner, various methods being used here as well, such as, inparticular, seal-glass bonding, wafer direct bonding, or anodic bonding.It is also provided in accordance with the present invention to bond orsolder first substrate 20 to second substrate 30. These bondingprocesses are mostly carried out in a vacuum. The reason for this isthat it is necessary for a vacuum to prevail in hollow space 10 after itis hermetically sealed. Often disadvantageously associated with thebonding processes is, however, that they, by nature, are oftenthemselves associated, with an evolution of gas, so that the internalpressure of hollow space 10 cannot be arbitrarily reduced. For thatreason, before hollow space 10 is sealed, in accordance with the presentinvention, a getter material is introduced into volume 10 to be occludedor contiguously thereto, the forming gases being bound by the gettermaterial, and the pressure prevailing in hollow space 10 being able tobe reduced. In accordance with the present invention, exclusively poroussilicon is provided as getter material. This porous silicon is shown inall figures as a region of porous silicon denoted by reference numeral11. In accordance with the present invention, when porous silicon isused as getter material, it is advantageous that only low temperaturesare needed to activate the getter property. Such low temperatures can beeasily integrated into semiconductor processes. This means that it ispossible to activate the porous silicon as getter material, even afterthe semiconductor components are manufactured, it being necessary toconsider that, typically, finished semiconductor components have a lowertemperature resistance than the pure semiconductor material used for itsmanufacture. The oxygen may be bound, for example, to the very largesurface area of porous silicon 11. Porous silicon dioxide is then formedfrom the gaseous oxygen and the porous silicon.

One particularly advantageous field of application of the methodaccording to the present invention and of the micromechanical componentaccording to the present invention is using the porous silicon as oxygengetter, for example during anodic bonding, the anodic bonding denoting abonding of silicon, in particular of silicon wafers, for the most partusing sodium-containing glass. In this process, oxygen is formed at thebonding surface, and diffuses into the hollow spaces and, in particular,into hollow space 10, and may be bound in hollow space 10, i.e., incavity 10 by getter materials, such as, in particular, by porous silicon11 according to the present invention, in order to produce a lowestpossible pressure in hollow space 10 or in cavity 10. The anodic bondingis carried out at an elevated temperature in accordance with the presentinvention of, for example 400° C., at which the porous silicon isreactive and at which the corresponding oxygen binds right in the sameprocess. The getter action of the porous silicon or of the region ofporous silicon 11 may be intensified in a subsequent optional annealingprocess.

In seal-glass bonding, as well, oxygen is formed, inter alia, which maybe bound, thereby enabling the internal pressure in hollow space 10 orin cavity 10 to be at least partially lowered. Other gases besidesoxygen, which are likewise formed during seal-glass bonding, may,however, likewise be adsorbed at the very large surface area of theporous silicon and, thus, also reduce the pressure.

The porous silicon may also be advantageously used in accordance withthe present invention as getter material in a silicon fusion bondingprocess, high temperatures of about 1000° C. being reached during thisprocess, the porous silicon not being destroyed, however.

FIG. 1 shows a diagrammatic sketch of the micromechanical componentaccording to the present invention. In an enclosed volume, alsodesignated as hollow space 10, at any desired surface area, i.e., at anydesired location adjoining hollow space 10, a region of porous silicon11 is provided, the micromechanical component according to the presentinvention having a first substrate 20 and a second substrate 30, firstsubstrate 20 being joined to second substrate 30 and an intermediatelayer 25 being provided between substrates 20, 30. In accordance withthe present invention, first substrate 20 is provided, in particular, asa silicon substrate, and second substrate 30 is likewise provided, inparticular, as a silicon substrate. In the diagrammatic sketch of FIG.1, the region of porous silicon 11 is provided in second substrate 30.When second substrate 30 is provided as the silicon substrate, region 11of porous silicon may be produced quite simply by etching into thesilicon material of second substrate 30. When bonding first substrate 20to second substrate 30, it is especially provided in accordance with thepresent invention to carry out a wafer bonding method or a wafer bondingmethod step, intermediate layer 25 being provided, for example, assealing glass.

FIG. 2 shows a first specific embodiment of the micromechanicalcomponent according to the present invention. The component includesfirst substrate 20, which has a micromechanical structure 200. Toproduce the micromechanical structure, a sacrificial layer 21, forexample in the form of a sacrificial oxide 21, is provided in firstsubstrate 20. A functional layer 22, constituted, in particular, of anepi-polysilicon layer, is also provided in first substrate 20 in thecomponent according to the present invention, to produce micromechanicalstructure 200. In this functional layer 22, micromechanical structure200 includes resonator structures, for example. Also shown in FIG. 2 issecond substrate 30, which includes region 11 of porous silicon. Whenfirst substrate 20 is joined to second substrate 30 at a bonding layer26, hollow space 10 is formed by the structuring of substrates 20, 30.The region of porous silicon 11, which adjoins hollow space 10, isprovided in second substrate 30. Micromechanical structure 200 forms, inparticular, an acceleration or rotation-rate sensor. Bonding layer 26includes, in particular, sealing glass.

FIG. 3 shows a second specific embodiment of the micromechanicalcomponent according to the present invention. In this case, firstsubstrate 20 is shown in the upper region of FIG. 3. First substrate 20is also referred to as sensor substrate. In first substrate or on firstsubstrate 20, micromechanical structure 200 is produced, to this end, asacrificial layer 21 and a functional layer 22, in particular ofepi-polysilicon, again being provided. In the second specificembodiment, a composite wafer is provided, in particular, as a secondsubstrate 30, the composite wafer being composed of actual secondsubstrate 30 and of another bonding layer 27, other bonding layer 27being joined, in particular, as a Pyrex glass layer, to actual secondsubstrate 30. In this respect, together with second substrate 30,additional bonding layer 27 forms the composite wafer. In the compositewafer, a region of porous silicon 11 is provided, as is a so-calledshield electrode 50. Shield electrode 50 is needed, in particular, toprotect the resonator structures of the functional layer during anodicbonding. In response to application of a voltage, which is required forthe subsequent anodic bonding, the resonator structures are pulledupwards by electrostatic forces and bonded to substrate 20. Shieldelectrode 50 is partially open, so that at the edge or at locationswhere no resonator structures of micromechanical structure 200 areprovided, a connection is established between hollow space 10 and theregion of porous silicon 11. This opening of the shield electrode isdenoted in FIG. 3 by reference numeral 51. Thus, volume 10 to beevacuated, i.e., hollow space 10 is joined to the region of poroussilicon 11.

FIG. 4 shows a third specific embodiment of the micromechanicalcomponent according to the present invention. In the third specificembodiment, the region of porous silicon 11 in first substrate 20 isproduced prior to application of sacrificial layer 21 or of functionallayer 22. The sequence of layers illustrated in FIG. 4 is derivedherefrom: initially first substrate 20, followed by the region of poroussilicon 11, (etched-away) sacrificial layer 21, and, subsequentlythereto, functional layer 22. The region of porous silicon 11, which isfirst produced in first substrate 20 in the third specific embodiment ofthe micromechanical component according to the present invention, isinitially covered when micromechanical structure 200 is produced, bysacrificial oxide 21, i.e., sacrificial layer 21, and subsequently alsoby functional layer 22. By etching away sacrificial layer 21 to uncovermicromechanical structure 200, the porous silicon, i.e., the region ofporous silicon 11 is exposed again and, thus, activated.

Both in FIG. 3, as well as in FIG. 4, i.e., in the second specificembodiment and in the third specific embodiment of the componentaccording to the present invention, it is provided very advantageouslyin accordance with the present invention for micromechanical structure200 to represent an acceleration sensor or a rotation-rate sensor, thisbeing provided, in particular, in the micropackage technology (MPT)illustrated in the figure.

FIG. 5 shows a fourth specific embodiment of the micromechanicalcomponent according to the present invention. The fourth specificembodiment represents an absolute-pressure sensor having porous siliconas getter material for producing a low internal pressure in hollow space10. The region of porous silicon is denoted, in turn, by referencenumeral 11 and is situated in first substrate 20. In addition, inaccordance with the fourth specific embodiment, the component accordingto the present invention includes a sacrificial layer 21 on firstsubstrate 20, upon which a membrane 60 is provided. The top side ofmembrane 60, which is shown in the top part of FIG. 5, is subject to theambient pressure of the pressure sensor, and an especially low internalpressure, produced by getter material 11, prevails in hollow space 10. Abending of membrane 60 as a function of the external pressure conditionsis detected by sensor elements (not shown in FIG. 5) in the area ofmembrane 60, and is converted into electrical signals.

1. A micromechanical component for a sensor, comprising: a body having afirst substrate and a second substrate that form a hollow space; and aregion of porous silicon located contiguously thereto, wherein theregion of porous silicon is provided for lowering a pressure prevailingin the hollow space, in that a gas is bound to the porous silicon. 2.The component as recited in claim 1, wherein the porous silicon bindsoxygen by forming silicon dioxide in response to a low temperature. 3.The component as recited in claim 1, wherein an intermediate layer isprovided between the first substrate and the second substrate, andwherein the porous silicon is in the second substrate.
 4. The componentas recited in claim 3, wherein the first substrate and the secondsubstrate are joined to one another in such a way that they arehermetically sealed at the intermediate layer.
 5. The component asrecited in claim 1, further comprising: a membrane, wherein the hollowspace is provided between the membrane and the first substrate, and theregion of porous silicon is provided in the first substrate.
 6. A methodfor manufacturing a sensor component, the method comprising: producing amicromechanical structure in a first substrate; producing in a secondsubstrate a region of porous silicon; joining the first substrate andthe second substrate; and lowering a pressure by activating the regionof porous silicon.
 7. A method for manufacturing a sensor component, themethod comprising: producing a region of porous silicon in a firstsubstrate; producing in the first substrate a micromechanical structure;and joining a second substrate to the first substrate; and lowering apressure by activating the region of porous silicon.
 8. A method formanufacturing a sensor component, the method comprising: producing aregion of porous silicon in a first substrate; producing in the firstsubstrate a micromechanical structure; and lowering a pressure byactivating the region of porous silicon.
 9. The micromechanicalcomponent of claim 1, wherein the micromechanical component is apressure sensor.
 10. The method of claim 6, wherein the micromechanicalstructure is a pressure sensor.
 11. A micromechanical pressure sensor,comprising: a first substrate; a second substrate, wherein the firstsubstrate is for bonding to the second substrate; and an intermediatelayer provided between the first substrate and the second; a body havinga hollow space and a region of porous silicon located contiguouslythereto, wherein the region of porous silicon is arranged to lower apressure prevailing in the hollow space by a gas being binded to theporous silicon.
 12. The sensor as recited in claim 11, wherein the gasis oxygen, and the porous silicon binds the oxygen by forming silicondioxide in response to a low temperature.
 13. The sensor as recited inclaim 11, wherein the first substrate and the second substrate arejoined to one another so that they are hermetically sealed at theintermediate layer.
 14. The sensor as recited in claim 11, wherein thehollow space is provided between the membrane and the first substrate,and the region of porous silicon is provided in the second substrate.15. The sensor as recited in claim 11, wherein the hollow space isprovided between the membrane and the first substrate, and the region ofporous silicon is provided in the first substrate.
 16. The sensor asrecited in claim 11, wherein the porous silicon is used to provide alarge surface area to provide effective getter characteristics.
 17. Thesensor as recited in claim 11, wherein the porous silicon is used toprovide a large surface area of up to about 1000 m² per cm³ to provideeffective getter characteristics.
 18. The sensor as recited in claim 11,wherein the porous silicon is used to eliminate the need for heating thecomponent to high temperatures to activate getter characteristics of theporous silicon.
 19. The component as recited in claim 1, wherein theporous silicon is used to provide a large surface area to provideeffective getter characteristics.
 20. The component as recited in claim1, wherein the porous silicon is used to provide a large surface area ofup to over 1000 m² per cm³ to provide effective getter characteristics.21. The component as recited in claim 1, wherein the porous silicon isused to eliminate the need for heating the component to hightemperatures to activate getter characteristics of the porous silicon.